Mechanism and improvement strategy of CoSe capacity change during lithiation/delithiation

Highlights: • The capacity of CoSe decreased first and then increased during charge/discharge. • The reduction in capacity is related to the amorphousness of the CoSe material. • The capacity increase is due to the recrystallization and the conductive SEI film. • The rGO significantly reduced the capacity changes of CoSe during charge/discharge. • The capacity of CoSe/rGO reaches 570 mA h g⁻¹ after 1000 cycles under 1000 mA g⁻¹. -- Abstract: Studying the reaction mechanism of materials in the process of lithiation/delithiation is important to understand and optimize the electrode materials. In this work, the petal-like CoSe is prepared by a simple hydrothermal method, and its lithium storage mechanism is studied. During the charge/discharge cycles, the capacity of the CoSe decreases first and then increases. In the initial stage, the volume of CoSe expands due to the intercalation of lithium ions, which results in the amorphousness of CoSe and reduces the specific capacity. The subsequent increase in capacity is due to the recrystallization of the material and the formation of a conductive SEI film. The petal-like CoSe displays a specific capacity of 450 mA h g⁻¹ at the current density of 100 mA g⁻¹ after 300 cycles. To improve the lithium storage performance, a CoSe/rGO composite is prepared. The addition of GO during the preparation of CoSe changes the morphology of CoSe from a larger petal shape to a smaller rod shape, which weakens the effect of volume change during lithium ion intercalation and shortens the lithium ion diffusion distance, so improves the reduction of specific capacity. At a current density of 100 mA g⁻¹, the specific capacity of CoSe/rGO composite can be as high as 730 mA h g⁻¹ after 200 cycles. Even under a large current density of 1000 mA g⁻¹, the specific capacity of the CoSe/rGO composite can still reach 570 mA h g⁻¹ after 1000 cycles.